L22 - Phosphate Flashcards
What does Pi mean?
Why is understanding how plants adapt to low Pi important?
- Pi = inorganic phosphate, (Po = organic phosphate)
- Rock Pi reserves for fertiliser are being exhausted
- Potential crisis
What is Phosphate used for in plants? Give 5 points.
Is it a macro or micro nutrient?
Structural and metabolic roles:
1) Energy donors ATP, PPi
2) Starch/sucrose synthesis
3) Phospholipids
4) DNA, RNA
5) Protein modification - phosphorylation
- Macronutrient?
Give a brief overview of the form of phosphate absorbed by plants
In what form phosphate is incorporated? Give 3 points
How it is stored and remobilised?
- Absorbed as oxidised anion - (which is?)
- Incorporated by phosphate esters containing one of:
1) CBH (C-O-P) (Cyclic Benzylidene Phosphate)
2) Pyrophosphate bonds (P-P)
3) Diester bridges (C-P-C) - Stored as polyphosphates in vacuole
- Remobilised as an anion
What can Pi deprivation cause in plants?
Give 6 things
- Low metabolism
- Stunted growth, chlorosis
- Delayed flowering
- Poor seed quality
- Leaf dropping
- Poor frost resistance (suboptimal membranes)
- Sulpholipids replace phospholipids
Describe how Pi is fixed in soils and what must happen for plants to absorb it.
As a result of this what do humans do?
- Pi fixed in soils, bound to Fe, Al or Ca
- Release of P to soil solution for absorption pH dependent
- Narrow “release” windows, acid + alkaline soils bad
- Pi usually limiting growth factor, fertiliser used w/ <10% uptake
Other than pH what factors are important for P release?
- Microbial phosphatase activity for Pi and Po release
- Healthy microbial populations essential
Why is phosphate acquisition difficult?
How do most species deal with this?
How does intensive agriculture threaten this?
- Available at 1-10µM in soil but needed in mM
- Low mobility (poor release)
- Depletion zone easily formed in roots
- 90% angiosperms reliant on mycorrhizal (myc) fungal symbioses
- Intensive agriculture = depleted myc (from tilling, monocultures etc…)
In the absence of mycorrhizal fungi how do plants adapt to low Pi?
Give 3 methods employed by different plants and describe each one
Changes in root system architecture (RSA):
Cluster roots form(tertiary laterals form on secondary lateral roots):
- Secrete organic acids = lower pH = more mobile Pi
- Also secrete phosphatases
Proteaceae take up Pi in winter for spring growth
Basal roots produced to exploit top soil Pi
- Risk of droughting
Why are Brassicas, which includes Arabidopsis a good model system for exploring Pi deprivation?
What responses do Brassica have to low Pi?
Give a specific Arabidopsis example
What causes these RSA changes?
- Never form myc symbioses
- Cessation of primary root growth
- Increased lateral root production
- Root hair numbers + length increased
- Normally alternation of epidermal cells for hair production in Arabidopsis but non-hair cells reprogrammed under deprivation
- Auxin causes changes
Give examples of the genes up-regulated in Arabidopsis and Rice in response to Pi deprivation
- 1/3 genes upregulated is transcription factor
- E.g. TF AtPHR1 (PHosphate Starvation Response) up-regulates root PM HAT H symporters AtPht1.1 and AtPht1.4.
- AtPHO1 upregulated to aid AtPHT1,2 load xylem
- PHT1s conserved in rice
- OsPHT1,9 and OsPHT1,10 support Pi uptake in deprivation
- Overexpression = increased Pi content
Give four gene families of transporters used for Pi distribution.
What must future studies target?
PHT2 family - plastid carriers
PHT3 family - mitochondria
PHT4 family - mostly plastid envelope
pPT family - plastidic phosphate translocators, inner envelope
- Which are upregulated under deprivation?
How are Pi levels maintained within cells during Pi deprivation?
Name the proteins responsible for this in rice and the evidence for this
- Pi released from Pi accumulations in the vacuole
- VPE1 and VPE2 transporters efflux Pi to cytosol in rice under deprivation
- Vacuolar retention increased in poor growing mutant
What mobilisation response can severe Pi deprivation cause in plants?
Give an example experimental set up that can explore nutrient remobilisation
Mobilisation from shoots to roots
- Cytokinin production in roots lowered
- Shoot Pi and sucrose sent to root
Split root experiments
- E.g. high affinity transporters not induced in Pi-deprived root section as Pi absorbed from replete section re-allocated via shoot
How do Phosphate plant levels also depend of the Sulphur levels of the plant?
Why are some areas in Europe now experiencing S deprivation?
- Under P deprivation phospholipids replaced by sulpholipids, freeing up P
- S deprivation worsens P deprivation
- Previously, industrial pollution + sulphuric acid in NPK fertiliser aided S levels in plants
- Better air quality and no S acid = S deprivation in some areas
Describe the following:
1) How is S taken up?
2) How does this change under S starvation?
3) Where is S transported to?
4) How is it assimilated?
5) Where is it stored?
1) As SO4(2-) by LAT and HAT H+ symporters
2) More HAT symporters induced
3) Transported into Chloroplasts
4) Assimilated (in chloroplasts) into cysteine, also makes glutathione (for maintaining redox balance)
5) Stored in vacuole under replete conditions (as cysteine, glutathione, SO4(2-) and more)
